As can be seen in FIG. 1, the prior art lifter bar 20 is typically attached to a rotatable mill shell 22 of a grinding mill by an attachment assembly 24. The lifter bar 20 is part of a lining 26 of the grinding mill, which typically includes a number of the lifter bars 20 and a number of shell plates. As shown in FIG. 1, the lifter bar 20 typically is positioned between two of the shell plates, identified in FIG. 1 by reference numerals 28A, 28B for convenience.
The lifter bars and shell plates may be made of various materials, and may include combinations of materials. For example, the lifter bars may be primarily made of a suitable rubber, a suitable composite, or a suitable steel. The prior art lifter bars may include various inserts or other parts made of different materials.
Typically, the lifter bar 20 includes a body 29 and a channel plate 30 (made of steel or aluminum, or any other suitable material) that extends along the length of the lifter bar 20. The channel plate 30 also defines a channel 32 in the lifter bar 20 in which part of the attachment assembly 24 is receivable.
The mill shell 22 includes a hole or slot 34 in which a bolt 36 is partially receivable. As can be seen in FIG. 1, the bolt 36 includes a head 38 at an inner end 39 of the bolt 36 that is engageable with an inner washer 40 that holds the head 38 in the channel 32. The bolt 36 extends between its inner end 39 and an outer end 42 thereof. As can also be seen in FIG. 1, the attachment assembly 24 typically also includes a nut 44 threadably engageable with the bolt 36 at the outer end 42. The nut 44, when tightened, urges an outer washer 46 against the mill shell 22, to subject the bolt 36 to tension, thereby pulling the lifter bar 20 outwardly (i.e., in the direction indicated by arrow “A” in FIG. 1), to secure the lifter bar in place inside the mill shell. (As will be described, the remainder of the drawings illustrate the present invention.)
The shell plates 28A, 28B include respective lip portions, identified by reference numerals 48A and 48B respectively in FIG. 1 for clarity of illustration. As can be seen in FIG. 1, the lip portions 48A, 48B are squeezed between the lifter bar 20 and the mill shell 22 when the nut 44 is tightened on the bolt 36.
In the arrangement illustrated in FIG. 1, the lifter bar body 29 is primarily made of rubber, and the shell plates 28A, 28B (including the lip portions 48A, 48B) are also made of rubber. The channel plate 30 preferably includes generally lateral extensions 50A, 50B that are pressed against portions 52A, 52B of the body 29 of the lifter bar 20 that are positioned between the lateral extensions 50A, 50B and the respective lip portions 48A, 48B. Where the body 29 is a suitable rubber or rubber composite, the channel plate 30 is initially held in place by vulcanization, i.e., by a chemical bond. However, once the attachment assembly 24 is tightened to secure the lifter bar 20 in place inside the mill shell 22, the lateral extensions 50A, 50B are also mechanically secured to the portions 52A, 52B of the body 29.
A charge (not shown) is positioned in the grinding mill, and the mill shell 22 is rotated, for comminution of pieces of ore in the charge that tumble against each other, and against the lining 26. As is well known in the art, the charge may include water and grinding media (e.g., balls, or rods). It will be understood that references herein to “solid parts of the charge” that collide with the lifter bar body include the pieces of ore, and where the charge includes grinding media, the solid parts of the charge may also include pieces of the grinding media.
As is known in the art, the body 29 that is at least partially rubber is at least somewhat resilient. The limited resilience of the at least partially rubber body 29 is thought to advantageously decrease the rate of wear of the body, because the resilience allows the lifter bar body 29 to absorb some of the dynamic impact of the solid parts of the charge colliding with it. However, as will be described, it appears that the resilience of the body 29 may result in premature failure of the bolt.
When in use, the mill shell 22 is rotated in the direction indicated by arrow “B”. As a result, the solid parts of the charge inside the mill shell 22, tumbling as the mill shell 22 is rotated in the direction indicated by arrow “B”, generally exert compressive force against the lifter bar 20 in the direction generally indicated by arrow “C”, i.e., due to dynamic loading of the solid parts of the charge on the lifter bar 20. It will be understood that the impacts of the tumbling solid parts of the charge on the lifter bar are multi-directional. Only one arrow (“C”) is used to represent the directions of the compressive forces dynamically exerted on the lifter bar by the tumbling solid parts of the charge, to simplify the illustration.
As illustrated in FIG. 1, at least some of the solid parts of the charge dynamically load the body in a direction that is at least partially transverse, e.g., such as schematically represented by arrow “C” in FIG. 1, and these may cause small pivoting movements of the lateral extensions. Also, the solid parts of the charge colliding with the body in other directions (e.g., in the direction indicated by arrow “A”) may cause small outward pivoting movements of the lateral extensions 50A, 50B. Over time, a relatively large number of collisions of the solid parts of the charge with the lifter bar occur.
The prior art has many disadvantages. In particular, in the prior art attachment assembly 24, the bolt 36 tends to break relatively frequently. The bolt 36 typically fractures in the region identified as “X” in FIG. 1. At this point, the mechanism of the failure of the bolt 36 is not well understood.
It is believed that the fracturing or rupturing of the bolt is the result of small rotational or pivoting movements of the lifter bar and the inner washer generally as indicated by arrow “D” in FIG. 1. In particular, it is thought that the solid parts of the charge that dynamically collide with the lifter bar (for example, as schematically represented by arrow “C”) urge the channel plate to pivot about a point on the bolt identified as “Y” in FIG. 1 (or a number of points), as schematically represented by arrow “D” in FIG. 1. Such rotational or pivoting movements, although initially small, are thought to gradually increase (due to repeated collisions of the solid parts of the charge with the body 29) until they are sufficient to subject the bolt to bending, and/or twisting (i.e., torque). It is believed that repeated bending (and/or twisting) of the bolt eventually results in metal fatigue, ultimately causing the bolt 36 to fail.
As is known in the art, the body 29 of the lifter bar 20 is primarily made of rubber formulated to have only limited resilience, so as to maximize its useful life. However, such rubber does have resilience, to a limited extent. As is well known in the art, the resilience of the rubber body 29 is believed to lessen the impact of the solid parts of the charge striking the body 29, thereby reducing wear and providing for a relatively longer useful life. The initial rotational movement of the channel plate 30 in response to the solid parts of the charge striking the lifter bar 20 is believed to be possible because the portions 52A, 52B of the body 29 of the lifter bar 20 are resilient, albeit to a limited extent only.
Similarly, the lip portions 48A, 48B typically are primarily made of the same or a similar rubber material as the lifter bar 20. The rotational movements are also thought to be permitted by the limited resilience of the lip portions 48A, 48B.
In general, due to the very high costs (i.e., because of lost production) associated with downtime of the grinding mill, it is critical that downtime be minimized. However, when the attachment assembly 24 no longer secures the lifter bar in position inside the mill shell (e.g., due to the bolt's failure), the grinding mill is taken off-line until repairs can be completed.